LOW & HIGH RESISTANCE MEASUREMENTS and A.C. BRIDGES

SECTION D LOW & HIGH RESISTANCE MEASUREMENTS: LIMITATIONS OF WHEATSTONE BRIDGE; KELVIN S DOUBLE BRIDGE METHOD, DIFFICULTIES IN HIGH RESISTANCE MEASUREMENTS. MEASUREMENT OF HIGH RESISTANCE BY DIRECT DEFLECTION, LOSS OF CHARGE METHOD, MEGOHM BRIDGE & MEGGAR.

A.C. BRIDGES: GENERAL BALANCE =N, CKT. DIAGRAM, PHASOR DIAGRAM, ADVANTAGES, DISADVANTAGES, APPLICATIONS OF MAXWELL S INDUCTANCE, INDUCANCE-CAPACITANCE, HAYS, ANDERSON, OWENS, DE- SAUTY S,SCHERING & WEINS BRIDGES, SHIELDING & EARTHING.

Low Resistance Measurement D.C Bridges: The basic D.C bridges consist of four resistive arms with a source of emf (a battery) and a null detector usually galvanometer or other sensitive current meter. D.C bridges are generally used for the measurement of resistance values.

Low Resistance Measurement Wheatstone Bridge: This is the best and commonest method of measuring medium resistance values in the range of 1Ω to the low megohm. The current through the galvanometer depends on potential difference between point (c) and (d). The bridge is said to be balance when potential difference across the galvanometer is zero volts, so there is no current through the galvanometer (Ig=0). This condition occurs when Vca=Vda or Vcb=Vdb hence the bridge is balance.

Kelvin s Bridge Kelvin bridge is a modification of the Wheatstone bridge and provides greatly increased accuracy in the measurement of low value resistance, generally below (1Ω). It is eliminate errors due to contact and leads resistance. (Ry) represent the resistance of the connecting lead from R3 to R4. Two galvanometer connections are possible, to point (m) or to point (n).

Kelvin s Bridge 1. If the galvanometer connect to point (m) then R4 = Rx + Ry therefore unknown resistance will be higher than its actual value by Ry 2. If the galvanometer connect to point (n) then R4 = Rx+ Ry therefore unknown resistance will be lower than its actual value by Ry.

Kelvin Bridge

Kelvin s Double Bridge Method

High Resistance Measurement Commonly used High Resistance measurement methods are- Direct Deflection Method Megohm Bridge MEGGAR

Direct Deflection Method

Megohm Bridge

MEGGAR

A.C.Bridges A.C.Bridges are those circuits which are used to measure the unknown resistances, capacitance, inductance, frequency and mutual inductance.

R 1 R 4 R 1 R 3 = R 2 R4 R 2 R 3

Generalized Bridge configuration Z 1.Z 4 =Z 2.Z 3

Components of AC bridges Four arms A source of excitation For low frequency measurement power-line acts as a source. For higher frequencies electronic oscillators are used. Battery Balance Detector Head Phone250 Hz to 3-4 khz Vibration Galvanometer 5 Hz to 1000Hz. But used below 200 Hz. Tuneable amplifier detector 10 Hz to 100 khz

Measurement of self- inductance

Maxwell s inductance Bridge In the Maxwell s inductance bridge,there are two pure resistances used for balance relations but on other side or arms the two known impedances are used. The known impedances and the resistances make the unknown impedances as Z1 and Z2.Such a network is known as Maxwell s A.C.. Bridge. As shown in fig.

In the Maxwell s inductance bridge,there are two pure resistances used for balance relations but on other side or arms the two known impedances are used. The known impedances and the resistances make the unknown impedances as Z1 and Z2.Such a network is known as Maxwell s A.C.. Bridge. As shown in figure below-

Fig. (R1 + jwl1 )R3 = (R4 + jwl4 )R2

Phasor diagram

Maxwell s inductance and capacitance bridge

Phasor Diagram

Anderson Bridge In the Anderson Bridge the unknown inductance is measured in terms of a known capacitance and resistance. this method is capable of precise measurements of inductance over a wide range of values from a few micro-henrys to several henrys and is the best bridge method.

Phasor diagram

Hay s Bridge It is particularly useful if the phase angle of the inductive impedance is large. In this case a comparatively smaller series resistance R1 is used instead of a parallel résistance.( which has to be of a very large value) as shown in fig.

Fig L3= C1 R2 R4 R3= w C1 R1 R2 R4 1+w R1 C1 1+w R1 C1

Measurement of Capacitance

Capacitance Bridge We will consider only De Sauty bridge method of comparing two capacitances the bridge has maximum sensitivity when C2 = C3. The simplicity of this method is offset by the impossibility of obtaining a perfect balance if both the capacitors are not free from the dielectric loss. A perfect balance can only be obtained if air capacitors are used. as shown in fig.

Desauty Bridge C2= C3 R1 /R2

Phasor Diagram E3=E4=I2R4 I1R3 E1=E2=I1/WC1

Schering Bridge Schering bridge used for the measurement of capacitance and dielectric loss of a capacitor. It is a device for comparing an imperfect capacitor C2 in terms of a loss-free standard capacitor C1. As shown in fig.

Schering bridge

Phasor Diagram

Wien Parallel Bridge It is also a ratio bridge used mainly as the feedback network in the wide range audio-frequency R-C oscillators. It is may be used for the measurement of the audio-frequency but it is not as accurate as the modern digital frequency meters. As shown in fig.

Wien s bridge

Sources of Errors in Bridge Circuits Stray conductance Mutual Inductance Stray Capacitance Residues in components

Precaution and Techniques used for reducing Errors Use of High-quality Components Bridge Lay-out Sensitivity Stray Conductance Effects Eddy Current Errors Residual Errors Frequency and Waveform Errors

Wagner Earthing Device